Results of earlier studies in this laboratory demonstrated that complexes between amino acids, carbon dioxide, chelators, and transition metals (iron or manganese) can catalyze the decomposition of hydrogen peroxide. The possibility that such complexes might serve an antioxidant function in vivo was supported by subsequent studies by other workers showing that the supplementation of the growth medium with glycine, manganese, and carbon dioxide protected endothelial cells in culture from hydrogen peroxide-induced toxicity, from injury associated with activation of neutrophils, and also from acute lung injury produced in vivo in rats by intratracheal instillation of a hydrogen peroxide-generating system. In the course of further studies in this laboratory, it was incidentally observed that when ferrozine was used as the metal chelator in such studies, the primary complex formed could be measured spectrophotometrically by its characteristic absorption spectrum. In an effort to study this potentially important phenomenon in greater detail, we have carried out in-depth studies on the chemistry of complex formation between ferrozine amino acids and ferrous iron, and in the ability of such complexes to scavenge reactive oxygen species. Based on a classical Job plot analysis, it was established that the reactive complex is composed of two equivalents of ferrozine, two equivalents of amino acid (histidine), and one equivalent of ferrous iron.